1. Field of the Invention
The present invention relates to an image reading apparatus and, more particularly, to an image reading apparatus which is adapted to read image information on an original surface with a line sensor (CCD) as an image reading means through an imaging means having a zoom lens by slit scanning and is suitable for an apparatus such as an image scanner.
2. Related Background Art
Conventionally, various image reading apparatuses for scanning an original surface by slit scanning, forming an image of a light beam based on image information on the original surface on the surface of a line sensor as an image reading means through an imaging lens, and digitally reading the image information with the line sensor have been proposed.
Most imaging lenses used in image reading apparatuses of this type are formed from single lenses. Image information on an original surface is read by a line sensor and then converted into a digital signal. Hence, enlargement and reduction can be easily performed by electronic zooming.
On the other hand, so-called analog copying machines for directly exposing image information on an original surface on, e.g., a photosensitive drum as a recording medium often use a zoom lens as an imaging lens for enlarging or reducing the image. The zoom lens of an analog copying machine has an aperture stop near the lens center, though the stop diameter is fixed in accordance with the zoom magnification.
In an image reading apparatus for digitally reading image information on an original surface using a line sensor by slit scanning, enlargement by electronic zooming limits read data. For this reason, the reproduced image is unclear. In addition, for reduction, image data is thinned to generate an unnatural portion in the reproduced image. Hence, to reproduce a clear enlarged image or a natural reduced image, the imaging lens is preferably formed from a zoom lens.
An image reading apparatus using a zoom lens for an image scanner is proposed in, e.g., Japanese Laid-Open Patent Application No. 9-113804. In this prior art, a first lens group having a negative refracting power and a second lens group having a positive refracting power are sequentially arranged from the object side. Zooming is realized by changing the distance between the first group and second group. However, characteristics such as the light amount in a peripheral portion, distortion, and the like in this prior art do not reach the level of a conventional image scanner using a single-focal-point lens.
For example, when the stop diameter is fixed for enlargement, the effective F-number of the zoom lens of an analog copying machine increases. However, this increase is canceled by an increase in exposure area due to enlargement of the slit image and the exposure amount becomes almost equivalent to that for an equi-magnification (one to one magnification). Hence, for an analog copying machine, the stop diameter is preferably fixed to obtain a constant scanning speed (image reading speed) of the photosensitive drum independently of zooming.
In a digital image reading apparatus, the light-receiving area of the line sensor does not change. For this reason, when the aperture stop diameter is fixed, the effective F-number changes upon zooming, and the light-receiving amount of the line sensor changes. Hence, to ensure an appropriate exposure amount in image reading, the scanning speed must be changed upon zooming.